The present invention relates to a method of manufacturing a plastic article with a cavity enclosed in the same, comprising the following steps: injection of plastic material under pressure into a mould cavity, cooling the plastic material closest to the walls of the mould cavity, injection of gas under pressure into fluent plastic material, which is surrounded by the cooled plastic material, pressing out, by means of the injected gas, fluent plastic material to at least one spill chamber located outside the mould cavity and communicating with the mould cavity injected gas, to form a cavity in the plastic material, and separation of the plastic material in the spill chamber from the plastic material in the mould cavity.
The invention also relates to an injection moulding device with a mould cavity defined by movable walls, comprising means for injecting under pressure fluent plastic material into the mould cavity, means for injecting gas under pressure into the plastic material in the mould cavity, at least one spill chamber located outside the mould cavity and communicating with the mould cavity, said spill chamber being arranged to receive plastic material forced out by the injected gas, and means for opening and cutting off the communication with the spill chamber.
The method of manufacturing hollow plastic articles by first filling the mould cavity completely as in normal injection moulding and thereafter, with the aid of the gas, pressing the core material out to a spill changer, so that the cavity in the article is formed, is usually called xe2x80x9cblow-out gas injectionxe2x80x9d and is described, for example, in U.S. Pat. No. 5,204,051. In the known method, the communication between the mould cavity and the spill chamber is kept closed during the injection of the plastic material and is opened after the mould cavity has been completely filled, and only after the surface of the plastic material has hardened somewhat. The purging here is done by leakage between the mould halves, and this means that the counter-pressure against the plastic material will be relatively great.
In this process, all of the plastic material injected into the mould cavity will remain in the cavity. This means that the plastic material, which, at the beginning of the injection, is in and nearest to the nozzle of the mould injector and which has a lower temperature and/or poorer quality than the rest of the plastic material, especially when the flow channel areas are large, can end up in the mould cavity at a location farthest away from the inlet to the mould cavity. This can occur in particular when injecting thermosetting resins and/or so-called cross-linkable plastics.
The purpose of the present invention is to achieve a method and a device, through which the above mentioned disadvantage can be eliminated, so that more even wall thickness and uniform plastic quality can be achieved in the manufactured plastic article.
This is achieved by virtue of the fact that a communication with a predetermined flowthrough area is maintained between the mould cavity and the spill chamber during the injection of the fluent plastic material, and that a communication with a larger predetermined flowthrough area is maintained between the mould cavity and the spill chamber during the injection of the gas.
The communication with smaller flowthrough area is maintained for such a long time that all the air is pressed out of the cavity and possibly a small amount of the plastic material is pressed out to the spill chamber. The method according to the invention achieves not only a product with an even wall thickness and uniform plastic quality but also makes it possible to shorten the production cycle by virtue of the fact that purging of the mould cavity of air can be done much more rapidly than when the purging is done by normal leakage between two mould halves. More rapid purging also reduces the risk of small air bubbles forming in the plastic material.
An injection moulding device for carrying out the method is characterized in that a communication with a predetermined flowthrough area is maintained between the mould cavity and the spill chamber during the injection of the fluent plastic material, and that a communication with a larger predetermined flowthrough area is maintained between the mould cavity and the spill chamber during the injection of the gas.